The present invention relates to electromagnetic tracking devices and, more particularly, to a system and method for tracking a medical probe such as a catheter as the probe is moved through the body of a patient.
It is known to track the position and orientation of a moving object with respect to a fixed frame of reference, by equipping the moving object with a transmitter that transmits electromagnetic radiation, placing a receiver in a known and fixed position in the fixed frame of reference, and inferring the continuously changing position and orientation of the object from signals transmitted by the transmitter and received by the receiver. Equivalently, by the principle of reciprocity, the moving object is equipped with a receiver, and a transmitter is placed in a known and fixed position in the fixed frame of reference. Typically, the transmitter includes three orthogonal magnetic dipole transmitting antennas; the receiver includes three orthogonal magnetic dipole receiving sensors; and the object is close enough to the stationary apparatus (transmitter or receiver), and the frequencies of the signals are sufficiently low, that the signals are near field signals. Also typically, the system used is a closed loop system: the receiver is hardwired to, and explicitly synchronized with, the transmitter. Representative prior art patents in this field include U.S. Pat. No. 4,287,809 and U.S. Pat. No. 4,394,831, to Egli et al.; U.S. Pat. No. 4,737,794, to Jones; U.S. Pat. No. 4,742,356, to Kuipers; U.S. Pat. No. 4,849,692, to Blood; and U.S. Pat. No. 5,347,289, to Elhardt. Several of the prior art patents, notably Jones, present non-iterative algorithms for computing the position and orientation of magnetic dipole transmitters with respect to magnetic dipole receivers.
An important variant of such systems is described in U.S. Pat. No. 5,600,330, to Blood. In Blood""s system, the transmitter is fixed in the fixed reference frame, and the receiver is attached to the moving object. Blood""s transmitting antennas are spatially extended, and so cannot be treated as point sources. Blood also presents an algorithm which allows the orientation, but not the position, of the receiver relative to the transmitter to be calculated non-iteratively.
Systems similar to Blood""s are useful for tracking a probe, such as a catheter or an endoscope, as that probe is moved through the body of a medical patient. It is particularly important in this application that the receiver be inside the probe and that the transmitter be external to the patient, because transmitting antennas of sufficient power would not fit inside the confined volume of the probe. A representative prior art system of this type is described in PCT Publication WO 96/05768, entitled xe2x80x9cMedical Diagnosis, Treatment and Imaging Systemsxe2x80x9d, which is incorporated by reference for all purposes as if fully set forth herein. Medical applications of such systems include cismyocardial revascularization, balloon catheterization, stent emplacement, electrical mapping of the heart and the insertion of nerve stimulation electrodes into the brain.
Perhaps the most important application of this tracking is to intrabody navigation, as described by Acker in U.S. Pat. No. 5,729,129, with reference to PCT Publication No. WO 95/09562. A three-dimensional image, such as a CT or MRI image, of the patient is acquired. This image includes fiducial markers at predetermined fiducial points on the surface of the patient. Auxiliary receivers similar to the receiver of the probe are placed at the fiducial points. The signals received by the auxiliary receivers are used to register the image with respect to the transmitter frame of reference, so that an icon that represents the probe can be displayed, superposed on a slice of the image, with the correct position and orientation with respect to the image. In this way, a physician can see the position and orientation of the probe with respect to the patient""s organs.
WO 96/05768 illustrates another constraint imposed on such systems by the small interior dimensions of the probe. In most prior art systems, for example, the system of Egli et al., the receiver sensors are three concentric orthogonal coils wound on a ferrite core. The coils are xe2x80x9cconcentricxe2x80x9d in the sense that their centers coincide. Such a receiver of sufficient sensitivity would not fit inside a medical probe. Therefore, the sensor coils of WO 96/05768 are collinear: the three orthogonal coils are positioned one behind the other, with their centers on the axis of the probe, as illustrated in FIG. 3 of WO 96/05768. This reduces the accuracy of the position and orientation measurements, because instead of sensing three independent magnetic field components at the same point in space, this receiver senses three independent magnetic field components at three different, albeit closely spaced, points in space.
A further, consequent concession of the system of WO 96/05768 to the small interior dimensions of a catheter is the use of coils wound on air cores, rather than the conventional ferrite cores. The high mutual coupling of collinear coils wound on ferrite cores and measuring three independent field components at three different points in space would distort those measurements sufficiently to make those measurements fatally nonrepresentative of measurements at a single point.
Another drawback of the system of WO 96/05768 relates to the geometry of the transmitter antennas. These are three nonoverlapping flat coplanar coils, preferably arranged in a triangle. Because the strength of the field transmitted by one of these coils falls as the reciprocal cube of the distance from the coil, the receiver usually senses fields of very disparate strength, which further degrades the accuracy of the position and orientation measurements. Acker addresses this problem by automatically boosting the power supplied to transmitting coils far from the receiver. In U.S. Pat. No. 5,752,513, Acker et al. address this problem by overlapping the coplanar transmitting coils.
Acker et al. transmit time-multiplexed DC signals. This time multiplexing slows down the measurement. Frequency multiplexing, as taught in WO 96/05768, overcomes this problem, but introduces a new problem insofar as the transmitting coils are coupled by mutual inductance at non-zero transmission frequency, so that the transmitted field geometry is not the simple geometry associated with a single coil, but the more complex geometry associated with several coupled coils. This complicates and slows down the calculation of the position and orientation of the receiver relative to the transmitter coils. PCT Publication WO 97/36143, entitled xe2x80x9cMutual Induction Correctionxe2x80x9d, addresses this problem by generating, at each transmitter coil, counter-fields that cancel the fields generated by the other transmitter coils.
A further source of slowness in calculating the position and orientation of the receiver is the iterative nature of the calculation required for a spatially extended transmitter. As noted above, Blood calculates the position of the receiver iteratively. Even in the DC case, Acker et al. calculate both the position and the orientation of the receiver iteratively.
There is thus a widely recognized need for, and it would be highly advantageous to have, a faster and more accurate method for tracking a medical probe inside the body of a patient.
According to the present invention there is provided a system for tracking a position and an orientation of a probe, including a plurality of first sensors, each of the first sensors for detecting a different component of a vector force field, each of the first sensors including two sensor elements disposed symmetrically about a common reference point in the probe, the first sensors being mounted inside the probe.
According to the present invention there is provided a method for determining a position and an orientation of an object with respect to a reference frame, including the steps of: (a) providing the object with three independent sensors of electromagnetic radiation; (b) providing three independent transmitting antennas of the electromagnetic radiation, each of the transmitting antennas having a fixed position in the reference frame, at least one of the transmitting antennas being spatially extended; (c) transmitting the electromagnetic radiation, using the transmitting antennas, a first of the transmitting antennas transmitting the electromagnetic radiation of a first spectrum, a second of the transmitting antennas transmitting the electromagnetic radiation of a second spectrum independent of the first spectrum, and a third of the transmitting antennas transmitting the electromagnetic radiation of a third spectrum independent of the first spectrum; (d) receiving signals corresponding to the electromagnetic radiation, at all three of the sensors, at a plurality of times, in synchrony with the transmitting of the electromagnetic radiation; and (e) inferring the position and the orientation of the object noniteratively from the signals.
According to the present invention there is provided a system for determining a position and an orientation of an object, including: (a) a plurality of at least partly overlapping transmitter antennas; (b) a mechanism for exciting the transmitter antennas to transmit electromagnetic radiation simultaneously, the electromagnetic radiation transmitted by each of the transmitter antennas having a different spectrum; (c) at least one electromagnetic field sensor, associated with the object, operative to produce signals corresponding to the electromagnetic radiation; and (d) a mechanism for inferring the position and the orientation of the object from the signals.
According to the present invention there is provided a system for determining a position and an orientation of an object, including: (a) a plurality of at least partly overlapping transmitter antennas; (b) a mechanism for exciting each of the transmitter antennas to transmit electromagnetic radiation of a certain single independent frequency and phase, the mechanism including, for each of the transmitter antennas, a mechanism for decoupling the each transmitter antenna from the electromagnetic radiation transmitted by every other transmitter antenna; (c) at least one electromagnetic field sensor, associated with the object, operative to produce signals corresponding to the electromagnetic radiation; and (d) a mechanism for inferring the position and the orientation of the object from the signals.
According to the present invention there is provided a catheter, including: (a) a housing having a transverse inner dimension of at most about two millimeters; and (b) at least one coil, wound about a solid core, mounted inside the housing.
According to the present invention there is provided a system for navigating a probe inside a body, including: (a) a receiver of electromagnetic radiation, inside the probe; (b) a device for acquiring an image of the body; and (c) a transmitter, of the electromagnetic radiation, including at least one antenna rigidly attached to the device so as to define a frame of reference that is fixed with respect to the device.
According to the present invention there is provided a system for navigating a probe inside a body, including: (a) a first receiver of electromagnetic radiation, inside the probe; (b) a device for acquiring an image of the body; and (c) a second receiver, of the electromagnetic radiation, rigidly attached to the device so as to define a frame of reference that is fixed with respect to the device.
According to the present invention there is provided a method of navigating a probe inside a body, including the steps of: (a) providing a device for acquiring an image of the body; (b) simultaneously: (i) acquiring the image of the body, and (ii) determining a position and orientation of the probe with respect to the image; and (c) displaying the image of the body with a representation of the probe superposed thereon according to the position and the orientation.
According to the present invention there is provided a device for sensing an electromagnetic field at a point, including at least four sensing elements, at least two of the sensing elements being disposed eccentrically with respect to the point.
According to the present invention there is provided a method for determining a position and an orientation of an object with respect to a reference frame, including the steps of: (a) providing the object with three independent sensors of electromagnetic radiation; (b) providing three independent transmitting antennas of the electromagnetic radiation, each of the transmitting antennas having a fixed position in the reference frame, at least one of the transmitting antennas being spatially extended; (c) transmitting the electromagnetic radiation, using the transmitting antennas, a first of the transmitting antennas transmitting the electromagnetic radiation of a first spectrum, a second of the transmitting antennas transmitting the electromagnetic radiation of a second spectrum independent of the first spectrum, and a third of the transmitting antennas transmitting the electromagnetic radiation of a third spectrum independent of the first spectrum; (d) receiving signals corresponding to the electromagnetic radiation, at all three of the sensors, at a plurality of times, in synchrony with the transmitting of the electromagnetic radiation; (e) setting up an overdetermined set of linear equations relating the signals to a set of amplitudes, there being, for each of the sensors: for each transmitting antenna: one of the amplitudes; and (f) solving the set of linear equations for the amplitudes.
According to the present invention there is provided a method of navigating a probe inside a body, including the steps of: (a) providing a device for acquiring an image of the body; (b) simultaneously: (i) acquiring the image of the body, and (ii) determining a position and an orientation of the body with respect to the image; (c) determining a position and an orientation of the probe with respect to the body; and (d) displaying the image of the body with a representation of the probe superposed thereon according to both of the positions and both of the orientations.
According to the present invention there is provided a device for sensing an electromagnetic field at a point, including: (a) two sensing elements, each of the sensing elements including a first lead and a second lead, the first leads being electrically connected to each other and to ground; and (b) a differential amplifier, each of the second leads being electrically connected to a different input of the differential amplifier.
According to the present invention there is provided a catheter including: (a) an outer sleeve having an end; (b) an inner sleeve having an end and slidably mounted within the outer sleeve; (c) a first flexible member connecting the end of the outer sleeve to the end of the inner sleeve; and (d) a first coil mounted on the first flexible member.
According to the present invention there is provided a system for determining a position and an orientation of an object, including:(a) at least one transmitter antenna for transmitting an electromagnetic field; (b) a first electromagnetic field sensor, associated with the object and including two sensing elements responsive to a first component of the transmitted electromagnetic field, each of the sensing elements including a first lead and a second lead, the first leads being electrically connected to each other and to ground; and (c) a first differential amplifier, each of the second leads being electrically connected to a different input of the first differential amplifier.
According to the present invention there is provided an imaging device, including: (a) an electrically conducting surface; (b) a magnetically permeable compensator; and (c) a mechanism for securing the compensator relative to the surface so as to substantially suppress a distortion of an external electromagnetic field caused by the surface.
According to the present invention there is provided a device for sensing an electromagnetic field, including: (a) a housing, including a first pair of diametrically opposed apertures, (b) a first core mounted in the first pair of apertures; and (c) a first coil of electrically conductive wire wound about the core.
According to the present invention there is provided a probe for interacting with a body cavity, including: (a) a substantially cylindrical catheter; (b) a satellite; and (c) a mechanism for reversibly securing the satellite at a fixed position and orientation relative to the catheter after the catheter and the satellite have been inserted into the body cavity.
Each receiver sensor of the present invention includes two sensor elements placed symmetrically with respect to a reference point inside the probe. All the sensor element pairs share the same reference point, so that the measured magnetic field components are representative of the field component values at the single reference point, instead of at three different points, as in the prior art system, despite the confined transverse interior dimensions of the probe. Because of the symmetric disposition of the sensor elements with respect to the reference point, the measured magnetic field components are representative of the field components at the reference point, despite the individual sensing elements not being centered on the reference point. This property of not being centered on the reference point is termed herein an eccentric disposition with respect to the reference point.
In one preferred embodiment of the receiver of the present invention, the sensor elements are helical coils. Within each sensor, the coils are mutually parallel and connected in series. As in the case of the prior art receivers, the coils are arranged with their centers on the axis of the probe. To ensure that coils of different sensors are mutually perpendicular, the probe housing includes mutually perpendicular pairs of diametrically opposed apertures formed therein, the coils whose axes are perpendicular to the axis of the probe are wound about cores whose ends extend past the ends of the respective coils, and the ends of the cores are mounted in their respective apertures.
In another preferred embodiment of the receiver of the present invention, with three sensors, the sensor elements are flat rectangular coils bent to conform to the shape of the cylindrical interior surface of the probe. The sensor elements of the three sensors are interleaved around the cylindrical surface. The advantage of this preferred embodiment over the first preferred embodiment is that this preferred embodiment leaves room within the probe for the insertion of other medical apparati.
As noted above, within any one sensor, the coils are connected in series. This connection is grounded. The other end of each coil is connected, by one wire of a twisted pair of wires, to a different input of a differential amplifier.
In a preferred embodiment of a cardiac catheter that incorporates a receiver of the present invention, the catheter includes an inner sleeve mounted slidably within an outer sleeve. One of the sensors includes two coils mounted within the inner sleeve, towards the distal end of the catheter. The distal end of the inner sleeve is connected to the distal end of the outer sleeve by flexible strips. Each of the other sensors includes two coils mounted on opposed lateral edges of a pair of flexible strips that flank the inner sleeve, with the inner sleeve running between the two members of the pair. When the inner sleeve is in the extended position thereof relative to the outer sleeve, the flexible strips lie flat against the inner sleeve, and the catheter can be maneuvered towards a patient""s heart via the patient""s blood vessels. When the end of the catheter has been introduced to the targeted chamber of the heart, the inner sleeve is withdrawn to the retracted position thereof relative to the outer sleeve, and the pairs of flexible strips form circles that are concentric with the reference point. Also mounted on the outward-facing surfaces of the flexible strips and, optionally, on the distal end of the inner sleeve, are electrodes for electrophysiologic mapping of the heart. Alternatively, the electrode on the distal end of the inner sleeve may be used for ablation of cardiac tissue, for example in the treatment of ventricular tachycardia.
An alternative preferred embodiment of the cardiac catheter of the present invention has an inflatable balloon connecting the distal ends of the inner and outer sleeves. The coils of the external sensors are mounted on the external surface of the balloon. When the inner sleeve is in the extended position thereof relative to the outer sleeve, the balloon lies flat against the inner sleeve, and the catheter can be maneuvered towards the patient""s heart via the patient""s blood vessels. When the end of the catheter has been introduced to the targeted chamber of the heart, the inner sleeve is withdrawn to the retracted position thereof relative to the outer sleeve, and the balloon is inflated to a sphere that is concentric with the reference point.
Although the primary application of the receiver of the present invention is to tracking a probe by receiving externally generated electromagnetic radiation, the scope of the present invention includes receivers for similar tracking based on the reception of any externally generated vector force field, for example, a time varying isotropic elastic field.
The algorithm of the present invention for inferring the position and orientation of the receiver with respect to the transmitter is similar to the algorithm described in U.S. Pat. No. 6,188,355, to Gilboa. The signals received by the receiver are transformed to a 3xc3x973 matrix M. The columns of M correspond to linear combinations of the amplitudes of the transmitted fields. The columns of M correspond to the receiver sensors. A rotationally invariant 3xc3x973 position matrix W and a 3xc3x973 rotation matrix T are inferred noniteratively from the matrix M. The Euler angles that represent the orientation of the receiver relative to the transmitter antennas are calculated noniteratively from the elements of T, and the Cartesian coordinates of the receiver relative to the transmitter antennas are calculated from the elements of W. A preliminary calibration of the system, either by explicitly measuring the signals received by the receiver sensors at a succession of positions and orientations of the receiver, or by theoretically predicting these signals at the successive positions and orientations of the receiver is used to determine polynomial coefficients that are used in the noniterative calculation of the Euler angles and the Cartesian coordinates. In essence, the extra time associated With an iterative calculation is exchanged for the extra time associated With an initial calibration. One simplification of the algorithm of the present invention, as compared to the algorithm of U.S. Pat. No. 6,188,355, derives from the fact that the system of the present invention is a closed loop system.
The preferred arrangement of the transmitter antennas of the present invention is as a set of flat, substantially coplanar coils that at least partially overlap. Unlike the preferred arrangement of Acker et al., it is not necessary that every coil overlap every other coil, as long as each coil overlaps at least one other coil. The most preferred arrangement of the transmitter antennas of the present invention consists of three antennas. Two of the antennas are adjacent and define a perimeter. The third antenna partly follows the perimeter and partly overlaps the first two antennas. The elements of the first column of M are sums of field amplitudes imputed to the first two antennas. The elements of the second column of M are differences of field amplitudes imputed to the first two antennas. The elements of the third column of M are linear combinations of the field amplitudes imputed to all three antennas that correspond to differences between the field amplitudes imputed to the third antenna and the field amplitudes that would be imputed to a fourth antenna that overlaps the portion of the first two antennas not overlapped by the third antenna.
The signals transmitted by the various antennas of the present invention have different, independent spectra. The term xe2x80x9cspectrumxe2x80x9d, as used herein, encompasses both the amplitude and the phase of the transmitted signal, as a function of frequency. So, for example, if one antenna transmits a signal proportional to cos xcfx89t and another antenna transmits a signal proportional to sin xcfx89t, the two signals are said to have independent frequency spectra because their phases differ, even though their amplitude spectra both are proportional to xcex4(xcfx89). The term xe2x80x9cindependent spectraxe2x80x9d, as used herein, means that one spectrum is not proportional to another spectrum. So, for example, if one antenna transmits a signal equal to cos xcfx89t and another antenna transmits a signal equal to 2 cos xcfx89t, the spectra of the two signals are not independent. Although the scope of the present invention includes independent transmitted signals that differ only in phase, and not in frequency, the examples given below are restricted to independent transmitted signals that differ in their frequency content.
The method employed by the present invention to decouple the transmitting antennas, thereby allowing each antennas to transmit at only a single frequency different from the frequencies at which the other antennas transmit, or, alternatively, allowing two antennas to transmit at a single frequency but with a predetermined phase relationship between the two signals, is to drive the antennas with circuitry that makes each antenna appear to the fields transmitted by the other antennas as an open circuit. To accomplish this, the driving circuitry of the present invention includes active circuit elements such as differential amplifiers, unlike the driving circuitry of the prior art, which includes only passive elements such as capacitors and resistors. By xe2x80x9cdriving circuitryxe2x80x9d is meant the circuitry that imposes a current of a desired transmission spectrum on an antenna, and not, for example, circuitry such as that described in WO 97/36143 whose function is to detect transmissions by other antennas with other spectra and generate compensatory currents.
With respect to intrabody navigation, the scope of the present invention includes the simultaneous acquisition and display of an image of the patient and superposition on that display of a representation of a probe inside the patient, with the representation positioned and oriented with respect to the image in the same way as the probe is positioned and oriented with respect to the patient. This is accomplished by positioning and orienting the imaging device with respect to the frame of reference of the transmitter, in one of two ways. Either the transmitter antennas are attached rigidly to the imaging device, or a second receiver is attached rigidly to the imaging device and the position and orientation of the imaging device with respect to the transmitter are determined in the same way as the position and orientation of the probe with respect to the transmitter are determined. This eliminates the need for fiducial points and fiducial markers. The scope of the present invention includes both 2D and 3D images, and includes imaging modalities such as CT, MRI, ultrasound and fluoroscopy. Medical applications to which the present invention is particularly suited include transesophageal echocardiography, intravascular ultrasound and intracardial ultrasound. In the context of intrabody navigation, the term xe2x80x9cimagexe2x80x9d as used herein refers to an image of the interior of the patient""s body, and not to an image of the patient""s exterior.
Under certain circumstances, the present invention facilitates intrabody navigation even if the image is acquired before the probe is navigated through the patient""s body with reference to the image. A third receiver is attached rigidly to the limb of the patient to which the medical procedure is to be applied. During image acquisition, the position and orientation of the third receiver with respect to the imaging device is determined as described above. This determines the position and orientation of the limb with respect to the image. Subsequently, while the probe is being moved through the limb, the position and orientation of the probe with respect to the limb is determined using the second method described above to position and orient the probe with respect to the imaging device during simultaneous imaging and navigation. Given the position and orientation of the probe with respect to the limb and the orientation and position of the limb with respect to the image, it is trivial to infer the position and orientation of the probe with respect to the image.
Many imaging devices used in conjunction with the present invention include electrically conducting surfaces. One important example of such an imaging device is a fluoroscope, whose image intensifier has an electrically conducting front face. According to the present invention, the imaging device is provided with a magnetically permeable compensator to suppress distortion of the electromagnetic field near the electrically conducting surface as a consequence of eddy currents induced in the electrically conducting surface by the electromagnetic waves transmitted by the transmitting antennas of the present invention.
The scope of the present invention includes a scheme for retrofitting an apparatus such as the receiver of the present invention to a catheter to produce an upgraded probe for investigating or treating a body cavity of a patient. A tether provides a loose mechanical connection between the apparatus and the catheter while the apparatus and the catheter are inserted into the patient. When the apparatus and the catheter reach targeted body cavity, the tether is withdrawn to pull the apparatus into a pocket on the catheter. The pocket holds the apparatus in a fixed position and orientation relative to the catheter.